Refractory lined centrifugal casting molds



Nov. 19, 1963 E. H. PHELPS REFRACTORY LINED CENTRIFUGAL CASTING MOLDS Filed April 5, 1961 INTRODUCE INTO MOLD HAVING WALL TEMPERATURE LESS THAN 2I2F. A MEASURED QUANTITY OF FLOWABLE SLURRY OF NG AGENT AND LIQUID SAND,SIL ICA FLOUR,SUSPENDI SUFFICIENT To FORM LINING AT LEAST .O25"IN THICKNESS AT SPEED SUFFICIENT TO LY DISTRIBUTE SLURRY OVER OF MOLD WALL AND FORM A LINING ROTATE MOLD CENTRIFUGAL INNER SURFACE ON OF MOLD ATINCREASED SPEED UNTIL MAJOR PORTIONS OF SILICA FLOUR,SUSPENDING AGENT AND LIQUID MOVE TO INNER PORTION OF LINING AND SAND COMPACTS AGAINST INNER SURFACE OF MOLD WALL CONTINUE ROTATI MOLD DRAIN FREE LIQUID FROM WASH INNER SURFACE OF LINING TO REMOVE FREE RESIDUAL SUSPENDING AGENT PASSING HEATED AIR THROUGH INTERIOR OF 4 TO 8 HOURS,TEMPERATURE OF AIR NOT F. DURING FIRST HOUR AND INCREAS- TO FROM 400 F. TO 600' F.

DRY LINING BY MOLD FOR FROM EXCEEDING 300 ING THEREAFTER INVENT OR EDWIN H PHELPS BY 2 1/ y ATTORNEYS United States Patent 3,110,944 REFRACTORY LINED CENTRIFUGAL CASTING MOLDS Edwin H. Phelps, Birmingham, Ala., assignor to American Cast Iron Pipe Company, Birmingham, Ala., a

corporation of Georgia Filed Apr. 5, 1961, Ser. No. 100,810 11 Claims. (Cl. 22-192) This invention relates to refractory lined molds for use in the centrifugal casting of tubular metal articles, such as cast iron and steel pipes and tubing, and is particularly directed to a new method of forming sand linings of substantial thickness and high density in cylindrical metal molds.

The molds to which the present invention relates cornprise sand linings having thicknesses in excess of about 0.25" which are customarily formed by ramming sand around a pattern while the latter is centered in a metal flask, and, in some instances, thereafter smoothing the inner or casting surface of the lining by means of a strike. This procedure is slow and uneconomical in the use of labor and materials, requires considerable special equipment, including patterns and flasks of various sizes, rammers and strikes, and cannot be relied upon to produce linings of uniform density, strength and hardness.

It is one of the principal objects of this invention to provide a new and improved method of preparing refractory lined centrifugal casting molds which is faster, requires less labor and equipment, and results in better quality castings than the procedures heretofore used in the foundry industry.

Another object is the provision of a novel procedure for lining metal molds, especially molds which are used for centrifugally casting heavy thick-walled steel tubes, wherein the mold lining material is introduced into a mold as a flowable slurry of novel composition, incorporating no special binders or other expensive materials, and is formed entirely by centrifugal force into a relatively thick, uniformly compacted lining of unusually high density having an exceptionally smooth, hard inner surface against which metal may ba cast.

A further object is to provide a method of the character described which produces a refractory lined mold of materially improved characteristics in comparison with those of the prior art, and which enables the same metal flask to be used for making castings of many different sizes.

These and other objects, including the provision of a procedure whereby lined molds may be prepared quickly regardless of their diameters and lengths, and which produces linings of uniform density, strength and smoothness of surface, will appear more fully as the detailed description of the invention proceeds. In this connection, since the invention is especially well adapted to the production of sand lined, vented metal molds for the centrifugal casting of iron and steel tubes and pipes, the following disclosure will be directed primarily to this particular application of the inventive concept. By so doing, however, it is not intended to limit the scope of the invention to this specific form of mold, because it will be obvious that it is equally applicable to molds of other types.

In general, the method of the present invention, which is illustrated by the block diagram flow sheet constituting ice the single FIGURE of the accompanying drawing, is characterized by the steps of introducing the mold lining material into a mold in the form of a liquid suspension or slurry of sand and silica fines, rotating the mold at a relatively high speed sumcient to compact the solid particles of the suspension against the mold wall in the form of an exceptionally dense, hard lining of uniform thickness wherein most of the silica fines are concentrated in a smooth surfaced layer at the inner portion of the lining and the coarser particles of sand come to rest in the outer portion adjacent the mold wall while most of the liquid is forced to and collects at the inner surface, draining or otherwise removing the free liquid thus collected, and thereafter drying the lining so as to remove as much as possible of the liquid retained therein after the lining has been formed. The liquid of the slurry imparts mobility to the solid particles of sand and silica fines during the mold spinning operation and acts like a lubricant so that particles of different sizes fit tightly against one another and interlock to a substantially greater extent, and thereby form a substantially denser lining, than would be the case if the lining material were centrifugally distributed in dry form as in the so-called loose sand methods of the prior art. The presence of the liquid also makes it possible for the coarse and fine particles to move relatively to one another and segregate according to size under the effect of centrifugal force, a result which is not attainable with the loose sand procedure.

Water is, for obvious reasons, the liquid usually used in forming the mold lining slurries of the present invention, and will be the only liquid referred to in the following description. However, various other liquids can be used, either in mixture with or in lieu of water, so long as the density of the liquid constituent of the slurry is less than that of the individual solid particles. For example, alcohol, kerosene, oil and liquid resins are among the other liquids which might be employed in practicing the invention.

Although the composition of the slurry of lining materials, the manner in which the slurry is introduced into the mold, and the speed and duration of the mold rotation may vary as hereinafter indicated, the basic features of the invention are (a) incorporation of sulficient water in the slurry to make it readily fiowable and to enable the solid particles of the suspension to move under the influence of centrifugal force and segregate according to particle size, with the larger particles of sand next to the mold wall and most of the silica fines in the inner portion of the lining which forms the casting surface thereof, (b) rotation of the mold at a sulficiently high speed to produce the centrifugal forces necessary to effect the desired segregative movement of the solid particles, and (c) collection of a major portion of the Water at the inner surface of the lining as a result of the centrifugal action. The resulting lining, after removal of the free water, is uniformly compact and stable under normal handling conditions, and, in spite of the absence of a conventional binder, retains a smooth, hard inner surface of silica fines even after the lining is completely dried.

The composition of the lining slurry is essentially a major proportion of relatively coarse silica sand, a minor proportion of silica fines and sufiicient water to form a flowable suspension of such stability that the solid particles do not settle prior to the high speed notation of the mold. In addition, a small amount of a material capable of acting as a suspending or dispersing agent is preferably included so as to insure maintenance of the refractory particles in suspension for a reasonable period of time after the slurry is formed. More specifically, the preferred slurry composition consists of at least 50% silica sand of various grain sizes, at least 50% of the particles being larger than sieve screen No. 50, from about 8% to about 30% silica flour, at least 50% of the particles being smaller than sieve screen No. 325, from about 0.5% to about 3.0% western (sodium) bentonite, and from about 12% to about 30% water, all percentages being by weight.

When the mold to be lined is of the vented type conventionally used with pneumatically rammed sand linings, the sand component of the slurry should include a substantial quantity of relatively coarse particles larger than sieve screen No. 20 which, when the mold is rotated, will move to the outermost portion of the lining and fill up the vent holes, thereby preventing the escape through said holes of any substantial amount of the lining mixture. For example, a quantity of AFS No. 9 silica sand equal to from about 10% to about 15% of the total slurry composition has been found suitable for lining molds having Va" diameter vent holes in the walls thereof.

If desired, relatively fine silica sand having an average grain size not larger than about AFS No. 125 may be substituted for all or part of the silica flour. The amount of silica flour or other fines in the slurry may be varied between the limits of from about 8% to about 30% in accordance with variations in the total thickness of the lining which is to be formed in the mold, the proportion of fines varying inversely to the lining thickness because, regardless of the total thickness of the lining, it is desirable to maintain at the inner surface thereof a dense, hard, smooth layer of silica flour or fines of a thickness not less than about 0.03". In most instances, it will be found that a silica flour layer of from 0.06 to 0.38" in thickness is satisfactory, although in large diameter molds with relatively thick linings the thickness of the layer of fines may be as great as 1.00".

The total thickness of the mold lining is determined primarily by the outside diameter of the casting which is to be made in the mold, it being possible to produce various sizes of castings in the same mold flask by simply varying the thickness of the mold lining. In practice, however, it has been found preferable to use a mold flask having an inside diameter which is at least 0.5" greater than the outside diameter of the casting, so that the thickness of the lining will be not less than about 0.25". Although theoretically there is no limitation on the maximum thickness of the linings formed in accordance with the present invention, practical considerations, such as the mold spinning time required to form the lining, suggest an upper limit of about 6.0".

Other suspending or dispersing materials may be used in the lining slurry instead of western bentonite, including certain clays, such as attapulgite, which are capable of acting as suspending agents, and such organic materials as methyl cellulose, sodium carboxymethylcellulose and bydroxyethylcellulose, and sodium alginate. Materials such as Glutrin, Goulac and sodium aluminate may also be added as thickeners. The proportion of western bentonite or other suspending agent in the slurry will vary within the limits of from about 0. to about 3.0% in dependence on the average size of the silica particles, including the fines, the larger the average particle size, the higher the percentage of suspending material required to keep the particles in suspension.

The proportion of water in the slurry may also vary within the above-indicated limits according to the average particle size of the sand and silica flour or fines, the coarser the particle size, the lower the percentage of water required. As a general rule, a slurry containing about 20% water will be found suitable. In each case,

however, the relative amounts of the various components of the lining mixture should be so balanced that, when the slurry has been placed or formed in the mold, it is capable of flowing readily and evenly without settling of the solid particles. For example, the fluidity of the slurry should be such that 200 cc. thereof will flow by gravity through an opening having a diameter of 0.7" in from 2 to 300 seconds, preferably in from 4 to 8 seconds.

The following Table I gives, by way of example, the compositions of several different slurries which have been used in the centrifugal formation of linings in vented molds in accordance with the present invention, it being understood that the percentages shown are based on the total weight of the slurry, and that the sand and silica flour or fines are in new, rather than reclaimed, condition:

TABLE I.-SLURRY COMPOSITIONS Slurry A Slllca sand.

AFS N0. 9 AFS No. 32

Total s.

The AFS grain fineness numbers in Table I indicate the average grain or particle size of the sand as determined by the American Fo-undrymens Society test procedure of sieve analysis. Table 11, below, sets forth the analyses of typical new sands and silica fines which can be used in carrying out the invention, including those of Table I.

TABLE II.SAND ANALYSIS Screen N0. AFb AFS AFS AFS AFS Silica No.8 No.9 No.32 No. 39 No.125 Flour percent percent percent percent percent percent 6 25.6 1.9 0.0 0.0 0.0 H.

The above figures represent the percentages by weight of each AFS number sand and silica flour which are retained on the various sieve screens. In this connection, it will be noted that the coarsest screen used in sieving the silica flour was screen No. 80, which was not used in analyzing the other sands, that screens Nos. 150 and 325 were also used for the silica flour but not for the other sands, and that screens Nos. and 270 were not used for the silica flour but were used for the other sands.

As indicated above, Table I refers to slurries composed of sands and silica flour or fines which have not been used previously in mold linings. However, in large scale commercial use of the present method, it is possible to effect substantial savings in the cost of the lining materials by reclaiming and reusing the sand and silica flour in the manner hereinafter described.

The reclaimed material, which is mixture of what were originally new sands and silica flour, and which will be referred to hereinafter as reclaim sand, will vary in its analysis depending upon the grain sizes and proportions of the materials originally introduced into the system and upon the amount of the starting material which is unreclaimed. A typical reclaim sand usable in forming slurries according to the present invention may have the following analysis:

Reclaim Sand Analysis A sand of this analysis Would have an AFS grain fineness number of approximately 80, and may not have the proper grain size distribution to produce a lining of the character desired, particularly as far as the amount of silica fines is concerned. Consequently, in preparing a slurry using this reclaim sand, it may be preferable to add both some new silioa flour and some new coarse sand, as well as the required amount of western bentonite or other suspending agent. For example, when using reclaim sand of the above analysis, a slurry of the following composition has been found satisfactory:

Slurry R Silica sand: Percent AFS No. 9 4.9 Reclaim 61.6

Total 66.5

Silica flour 14.6 Western bentonite 1.0 Water 17.9

In preparing a lined mold in accordance with the preferred procedure of the present invention, the first step is to select the size of mold flask to be used, given the size of the casting which is to be produced. From the dimensions of the mold fiask and the casting, the thickness and volume of the lining are readily computed. The next step is to selected the composition of the slurry to be used in forming the lining, having regard to such factors as the character of the metal to be cast, the thickness of the lining, the rotational speed range of the available mold spinning apparatus, and the length of time within which it is desired that the lining be formed.

Once the slurry composition has been established, the proper proportions of sand, silica fines, suspending material and Water are thoroughly mixed to form a relatively stable suspension, using any convenient mixing device. A measured quantity of the slurry is then introduced into the mold in any suitable manner, as by means of a hose or trough, preferably While the mold is stationary with its axis in a, horizontal or only slightly tilted position, and after it has been provided with annular stop-off plates which define the ends of the lining and prevent the lining material from flowing out of the mold. Since evaporation of the slurry water during formation of the lining is undesirable, in that the loss of water would reduce the segregative movement of the sand and silica fines, while the production of steam would rupture or create voids in the lining, the temperature of the inner surface of the mold wall should be less than 212 F., and preferably not more than about 180 F., at the time when the slurry is placed in the mold.

The quantity of slurry required to form the lining may be determined voluinetrically by applying to the computed volume of the finished lining a known shrinkage factor which is derived experimentally for each slurry composition under various mold spinning conditions. In general, compositions having relatively high percentages of solids will shrink less during the centrifugal lining operation than slurries which contain greater percentages of water; also, the larger the average particle size of the solid constituents of the slurry, the less the shrinkage. On an average, however, the lining compositions of the present invention have a shrinkage factor of about 20%.

When the slurry has been distributed uniformly throughout the length of the mold, rotation of the mold is begun and continued at a relatively low speed for a brief period of time until the vent holes in the wall of the mold become filled with and stopped up by the coarser particles of sand in the slurry. This initial rotation should continue for approximately 15 seconds at a speed such that the centrifugal force exerted on the particles of the slurry is only slightly above the force of gravity, e.g., about 1.5g where g represents the force of gravity. Alternatively, and particularly when screened vents are used, the slurry may be introduced into the mold while the mold is rotating, but in this event the speed of rotation should be maintained below that at which the slurry is picked up and held against the mold wall by centrifugal force until it has been distributed uniformly longitudinally of the mold.

After the initial period of low speed rotation, spinning of the mold is accelerated, either at a substantially uniform rate or in stages, until a speed is attained such that, due to the centrifugal forces created, the solid particles of the slurry are uniformly distributed over and firmly compacted against the inner surface of the mold wall and the major portion of the slurry water is forced to and collects at the inner surface of the lining. The speed at which the mold is spun and the duration of the high speed spinning are so controlled that the solid particles of the slurry move about, aided by the lubricating action of the water, and form themselves into a dense lining of uniform thickness wherein most of the silica flour or fines is concentrated in a well defined layer at the inner surface of the lining while the coarser particles of sand constitute a substantially thicker outer layer adjacent the mold wall. When the slurry contains a suspending agent such as western bentonite, a substantial proportion of the initial quantity thereof, usually about 25%, is carried by the slurry water to the inner surface of the lining where it either remains suspended in the water or is deposited on the surface of the layer of silica fines, while most of the remainder remains mixed with the silica fines in said layer. Normally, at least half of the total amount of water in the slurry collects freely at the inner surface of the lining by the end of the high speed spinning operation, the remainder being retained in the lining. In most cases, the water content of the silica fines layer after spinning is about 12.0%, while that of the sand layer varies from 6.5% to 11.5%.

In determining the mold spinning speed and the length of the spinning period, a number of different factors are involved. For example, the greater the percentage of western bentonite or other suspending agent in the slurry composition, the longer the spinning time required to effect the desired segregation of the solid particles of the lining and formation of a well defined inner layer of silica fines. By increasing the spinning speed and time, it is possible to decrease the amount of water retained in the lining. Consequently, since the greater surface area of a finer sand retains more moisture than a coarser sand, slurries containing higher proportions of fines require higher spinning speeds and longer spinning times in order to remove the same amounts of water. With any given slurry composition, the more rapid the acceleration during spinning, the more quickly is it possible a, 1 10, see

to form a concentrated inner layer of fines, and the thicker the layer will be during the same spinning time. Also, since the coarser sand particles settle faster, the spinning time may be less with a relatively coarse sand than with a finer sand. The spinning time is, of course, longer for the thicker linings.

Although the speed at which the mold is spun while the lining is being formed and the duration of that operation may thus vary in accordance with variations in the slurry composition, lining thickness and desired moisture content of the finished lining, the maximum spinning speed should be such that the centrifugal force created at the inner surface of the lining is not less than about 20g (it may be as high as 3003 or more, depending upon the capacity of the spinning equipment), while the total spinning time may range from about 0.5 to about 15.0 minutes. A spinning for from 2.0 to 4.0 minutes at a maximum speed such as to produce a centrifugal force equivalent to about 200g is typical of the method of the present invention. The following Table 111 sets forth a number of examples of spinning speeds and times in relation to the slurry composition, mold diameter and lining thickness.

the surface of the silica fines layer and remove therefrom any suspending agent which may have settled out of the slurry water and deposited itself on top of said layer. If any substantial amount of residual suspending agent should be left on the inner or casting surface of the mold lining until after the drying operation next to be described, objectionable roughness and cracking of said surface may result.

After the free slurry water and suspended solids have been removed from the mold and any residual suspending agent has been Washed from the inner surface of the lining, a rotary type swab may be passed through the interior of the mold so as to absorb and remove whatever small amount of the wash water may remain on the lining surface. The lining is then dried in order to remove as much as possible of the Water remaining therein, the objective being to render the lining practically bone dry by evaporating all water except that which has chemically combined with impurities in the sand and silica fines. If any significant amount of unco mbined water is permitted to remain in the lining at the time of casting in the mold, the heat of the casting metal will create steam which, due to the density of the lining, may

TABLE III.EXAMPLES OF MOLD LINING PROCEDURE INIold 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 i 16 Slurry composition D F A A B C A 13 C D F F E R R 110M diameter (in.) 22.19 22.19 18. 00 18. 00 18. 00 18.00 18.00 7.18 7.18 7.18 7. 1b 7.18 7.18 6. 23 9. 45 13. G

Lining thickness (111.):

and 0. 62 (1. 70 0, 72 1. 08 0. 72 0. 70 0. 70 0. 49 0. 4S 0. 55 0. 31 0. 47 0 50 0, 30 O. 500 0 8125 Silica flour 0.16 0.12 0.16 0.17 0.09 0.05 0. 08 0.11 0.11 0.05 0.06 0. 9 0. 09 .s 0 125 0.1575

Total l 0. 78 0. 82 0. 88 l. 0. 81 l). 75 0. 78 0. 60 0. 59 0. G0 0. 3T 0. 50 0 50 0. 0. 625 .000 Max. spinning speed (r.p.rn. 665 655 845 500 405 485 865 1,190 1, 280 1, 215 1, 200 1, 320 640 1, 000 1, 275 1,100 Max. centrifugal force (g), 130 125 165 55 50 05 175 120 140 125 145 150 80 200 200 Spinning time (IIllIL) 2. 2. T5 2. 25 2. 25 2. 25 2. 25 3. 25 3. 25 2. 75 2. (50 2. 50 2. 50 5. 25 3. 50 3. 00 3.00

The relationship between the speed of rotation of the mold and the centrifugal force created at the inner surface of the lining is expressed by the formula 70,500 where N is the speed of rotation in r.p.m., d is the inside diameter of the finished lining and F is the centrifugal force expressed in terms of g which represents the force of gravity. For example, if it is desired to produce a centrifugal force equivalent to 80g at the inner surface of a mold lining having an inside diameter of 10.00" when completed, the speed at which the mold should be rotated or, in round figures, 750 rpm. All of the spinning speed figures in Table III were obtained by use of the above formula and rounded off to the nearest 5 rpm.

After the lining has been formed centrifugally in the manner above described, the free water which has collected at the inner surface of the lining during the high speed rotation is removed, carrying with it whatever amounts of the suspending agent and silica fines may remain suspended therein. Removal of the free Water may be accomplished by elevating one end of the mold after it has stopped rotating and draining by gravity all the water that will flow over the stop-01f plate at the 0pposite end, after which said plate may be taken off to permit complete drainage, or it may be left in place and a suction used to take out the remaining water which collects next to the plate. It is also preferable to run fresh water into the mold at the elevated end after the slurry water has been drained, and to simultaneously rotate the mold at a relatively low speed, so as to wash not be able to escape outwardly to and through the vent holes in the mold wall rapidly enough to avoid penetration into the casting.

Although the lining may be dried in various ways, the preferred procedure of the present invention involves placing the lined mold in a suitable oven, circulating heated air through the interior of the mold for a period of from about 4 hours to about 12 hours, maintaining the temperature of the air at not more than about 250 F. during the first 3 or 4 hours of the period to avoid rapid formation of large quantities of steam which might rupture the lining, and thereafter increasing the temperature to from about 580 to about 640 F. for the remainder of the drying period. The maximum drying temperature may exceed 640 R, if desired, provided it is not elevated to the point where the lining material is burned or sintercd. The lining is thus dried relatively slowly from its inner surface. When the drying is completed, the lined mold is ready for use for centrifugally casting metal according to the same procedures as those conventionally practiced in conjunction with the sand lined molds of the prior art.

If desired, molds may be lined well in advance of the time when they are to be used for casting, in which event the drying operation is delayed until shortly before casting. It has been found that an interval of as long as two weeks may elapse between the centrifugal lining step and the drying step of the method of the present invention without detriment to the quality of the finished mold.

A remarkable characteristic of mold linings formed in the manner above described is that, in spite of the fact that the lining does not include any material having the properties of a conventional binder other than the small percentage of bentonite which remains in the lining and whatever clay may be present as an impurity in the sand, the inner layer of silica fines remains firmly united to the rest of the lining even after all of the unoornbined water has been removed, and provides a casting surface of exceptional smoothness and such hardness that it is capable of effectively resisting penetration by the casting metal.

Another characteristic of molds which are lined in accordance with the present method is that, in spite of its extremely high density and hardness, the lining can be readily removed from the mold flask after the casting has been stripped by simply running a stream of water through the mold, a fact which also enables economical reclamation of the lining material. Accordingly, it is customary in practicing the invention commercially to flush the mold lining out of the mold flask after the casting has been removed, to pass the stream of flushing water, sand, silica flour and whatever bentonite may have been retained in the lining through a screen to remove any agglomerated masses of sand or sintered pieces of the layer of silica flour against which the metal was cast, and to then pump the stream to a settling basin where the water is permitted to drain off, leaving the sand and silica fines ready for transportation to a drying site. After drying, the reclaimed material may be reused, supplemented by new sand and/or silica flour, if desired, for forming lining slurries in the manner previously described.

Although the lining slurries of the present invention preferably include a small amount of western bentonite or other suspending agent in order to prevent settling of the solid particles prior to the mold spinning operation, it is possible to omit the suspending material either by introducing the slurry into the mold immediately after it is mixed, or by mixing the slurry in the mold itself while the latter is rotating. For example, after the stopoff plates have been placed on the ends of the mold and the mold is positioned on the rollers of the spinning machine, the total amount of water required to form the slurry may be run into the mold, and then, while the mold is rotated at about 50 rpm, equivalent to a centrifugal force of less than 1.0 g, the sand and silica fines may be dumped into the water by means of a trough having a length substantially equal to the length of the mold which may be run into and out of the mold in any suitable manner. Rotation at this relatively low speed is continued until the slurry is thoroughly mixed, whereupon the spinning speed is increased in at least two stages, first, in order to fill up the vent holes in the mold wall with the coarser particles of sand, and, second, to fourn the lining centrifugally, in the manner previously described.

As pointed out above, the bentonite in the slurry serves primarily as a dispersing agent to prevent undesirably rapid settling of the sand and silica fines, and is not normally present in sufficient quantity to act as a binder, particularly because the centrifugal lining operation forces a substantial proportion of the bentonite to the inner surface of the lining whence it is removed either with the free slurry water or by washing.

As an illustration of how the present invention may be carried out in practice, let it be assumed that it is desired to centrifugally cast a steel tube having an outside diameter of 17.20" and a length l of 246" in a mold flask having A3" diameter vent holes in the wall thereof. Since the thickness of linings formed in accordance with the present method should be not less than about 0.25, and since, in the assumed case of a steel casting of the size specified, it is desirable that the metal be cast on a lining of substantial thickness, a mold flask having an inside diameter D of 20.04" will be found suitable. Although the outside diameter of the casting is to be 17.20" when finished, it is known that, due to shrinkage of the metal on cooling, the inside diameter d of the mold lining, i.e., the hot" diameter of the casting, must be 17.68 in order to produce a finished tube of the de- 10 sired size. Consequently, the thickness r of the mold lining should be /2(Du), or V2 (20.0417.68)"= 1.18".

The next step is to select the composition of the lining slurry and to compute the amount of slurry required to produce a lining of the desired thickness, i.e., 1.18". It has been found by experimentation that, in order to form a lining of such thickness in a large diameter mold flask which cannot be rotated economically at speeds in excess of about 800 to 900 rpm, a slurry of the following composition is satisfactory:

Silica sand: Percent AFS No. 9 12.4 AFS No. 32 45.6

Total 58.0

Silica flour 23.2

Western bentonite 0.7

Water 18.1

where V and V represent, respectively, the volumes of the slurry and the finished lining. The volume of the finished lining is, of course, obtained by the formula where I and I represent, respectively, the length and thickness of the mold lining, and D represents the inside diameter of the mold flask. In the present instance, where the mold lining is to be 246 long and 1.18 thick and the inside diameter of the mold flask is 20.04", substitution in the above formulas results in values of 17,200 cubic inches for the volume of the finished mold lining and 21,500 cubic inches for the volume of the slurry. Converting the latter figure into liquid measure, it is found that 93.1 gallons of slurry are required to form the lining.

In preparing the slurry, the sand and silica fines are put into an ordinary concrete mixer, or any other suitable mixing device. in dry form, but the western bentonite is preferably made up as a wash consisting of 8% bentonite and 92% Water. After the bentonite wash has been dumped into the mixer, the remainder of the Water required to form the slurry is added and the mixing operation is performed. Although thoroughly mixed slurries containing suitable quantities of suspending material may be left standing as long as 48 hours before use Without excessive settling of the refractory particles, it is preferable to mix the slurry not more than about 3 hours prior to the time when it is to be used.

After the slurry has been made and thoroughly mixed, the required quantity of 93.1 gallons thereof is preferably pumped into the mold flask through a hose While the mold is stationary with its axis in a slightly elevated position. At this time the temperature of. the mold wall should be less than 212 F., preferably not more than about 180 F. As soon as the slurry has distributed itself uniformly along the length of the mold, the mold is placed on the rollers of a conventional mold spinning machine and the latter is energized to rotate the mold for 15 seconds at about r.p.1n., a speed which produces a centrifugal force equal to approximately 1.4g. During this period, the coarser particles of sand in the slurry settle in and stop up the vent holdes in the mold wall and the slurry begins to distribute itself uniformly over the inner surface of the mold wall under the influence of centrifugal force. Rotation of the mold is then accelerated uniformly for another period of 15 seconds until the speed of rotation reaches 200 r.p.m., creating a centrifugal force equivalent to about g, whereupon the latter speed is maintained for 30 seconds. At the end of the first minute, the spinning speed is again uniformly accelerated during the ensuing 60 seconds to a speed of 450 r.p.m., which produces a centrifugal force equal to approximately SOg. After the mold is spun for another 30 seconds at the latter speed, it is again accelerated over a period of 30 seconds until a maximum speed of 800 rpm. is reached, at which the centrifugal force is equal to about 160g. It will be understood that, in each in stance, the value of the centrifugal force stated is that produced at points positioned 1.18" from the inner wall of the mold flask, i.e., at what will be the inner surface of the finished mold lining, computed according to the formula above set forth.

The mold is continuously rotated at the maximum speed of 800 rpm. for 2 minutes, thus making a total spinning time of 5 minutes overall. During this time, the particles of sand and silica flour are formed into a hard, dense lining having a uniform thickness of 1.18", the major portion of the silica flour being concentrated in a well defined layer approximately 0.1875" in thick ness at the inner surface of the lining. while about 50.0% of the original amount of water in the slurry is squeezed out of the solid portion of the lining and collects at the inner surface thereof, carrying with it a substantial portion of the bentonite.

After rotation of the mold has ceased, one end thereof is elevated to an angle of about 20 so that the collected free water and suspended bentonite may drain out by gravity. The mold is then returned to a horizontal position. fresh water is introduced through a hose, and the mold is slowly rotated so as to thoroughly wash the inner surface of the silica flour layer and remove any of the bentonite that may remain deposited thereon, after which one end is again elevated and the stop-oil plate at the other end is removed to permit the wash water to drain. When the draining and washing operations have been completed and the surface of the lining has been swabbed to remove Whatever small amount of wash water may remain thereon, the lined mold is transferred to a drying oven and dried with heated air for a period of about 8 hours. The temperature of the air is maintained between 230 and 250 F. during the first 3 hours of the drying period, is then raised to about 600 F. during the fourth hour, and is maintained between 580 and 640 F. during the last 4 hours. At the end of the 8 hour drying period. the mold lining is uniformly hard, dense, smooth and substantially bone-dry, and the mold is ready for casting.

Molds which have been lined by the method of the present invention may be used for casting any metal that may be cast in sand, including gray cast iron, alloy cast irons, plain carbon steels and alloy steels, and are particularly well adapted for the casting of steel tubing. Due to the presence of the layer of silica fines which forms at the inner surface of the mold lining, tubes and pipes cast in these molds have exceptionally smooth surfaces, the controlled thickness of said layer insuring against burn in" or metal penetration and effectively preventing fusion of the sand. As indicative of the value of such a lining, it may be noted that steel tubes up to 20 feet in ength and Weighing as much as 18,000 pounds have been cast in molds lined in accordance with the invention and no penetration has occurred, even where the metal impinged directly on the mold lining during pouring. On the other hand, similarly prepared molds have been used for castings as short as 12" and weighing only a few pounds.

While the method of the present invention is adapted primarily for the lining of vented mold flasks, it may also be applied to the lining of solid metal molds as long as the finished lining contains no gas-forming materials, such as bentonite. In such cases, or when the vent holes are provided with screens, the average particle size of the sand used in the slurry may be finer than otherwise, since there is no need for the presence of relatively coarse particles to fill up the vent holes. Furthermore, although the method is particularly applicable to molds for casting straight tubes and pipes, molds for tubular a1 ticlcs of irregular shape, such as pipes having bells and beads, may be readily prepared by the same method by simply placing appropriate cores or patterns within the mold flasks and letting the slurry flow to form whatever shape is desired.

It will be evident from the foregoing disclosure that the present method of lining metal molds for use in the centrifugal casting of tubular metal articles possesses a number of advantages in comparison with the procedures of the prior art for preparing sand lined molds. For example, although this procedure utilizes the same metal mold flasks as those heretofore used with pneumatically rammed sand linings, substantially less labor, material and equipment are required to prepare the molds. Using conventional foundry techniques, as much as 1 /2 man hours may be required to form a pneumatically rammed sand lining in a vented metal mold, whereas the same mold may be lined by the present method in 10 minutes, both times being exclusive of drying. The average cost of the sand, silica hour and bentonite forming the slurry used for lining a mold to produce a dentrifugully cast tube of any given diameter is less than half the cost of the materials required to line a rammed sand mold for a casting of the same size.

The use of a water slurry containing sand and silica fines of various grain sizes and of high spinning speeds for ccntrifugally forming the lining imparts mobility to the lining material, insures a uniformly compacted lining without the use of rammers, and produces such a smooth lining that no patterns or strikes are required to smooth the casting surface. Linings of greater density are also provided by this method, the density of centrifugally formed slurry linings being from about pounds to about pounds per cubic foot, whereas the maximum density of rammed linings is about 110 pounds per cubic foot. As a result of the uniform compaction and greater density of linings formed in accordance with the invention, casting defects known as buckles are eliminated, there is less tendency for the castings to tear or crack during solidification, and the mold linings expand and contract at a uniform rate with no tendency to become enlarged at any point because of Weak areas. The increased density of the linings also permits the use of higher spinning speeds in centrifugally casting tubing than is possible with molds of the prior art, which in turn results in sounder sections requiring less stock removal and in smoother inside surfaces.

Another important advantage of the present method is that the same mold flask may be used to produce castings of different diameters by simply varying the amount of slurry used and thereby varying the thickness of the lining, but without requiring the use of lining patterns of different sizes individually related to the size of the casting as is necessary in forming rammed sand linings. Furthermore, castings of different lengths may be made in the same mold flask by placing an oil sand stop-off core in the flask at the proper point along the length thereof before the mold is lined, and then lining the mold on both sides of the core in the manner above described, whereby the two sections of the finished lining effectively maintain the core in position during the casting operation so as to limit the length of the casting as desired.

Although a number of examples have been given of specific slurry compositions and spinning procedures usable in performing the method of the present invention, it is to be expressly understood that these data are illustrative only and are not to be construed as representing the full scope of the invention. Since it is evident that various changes, which will now suggest themselves to those skilled in the art, may be made in the composition of the slurry, the speed and duration of the mold spinning operation and the other details of the method, reference should be had to the appended claims for a definition of the limits of the invention.

This application is a continuation-in-part of application Serial No. 764,779, filed October 2, 1958, and now abandoned.

What is claimed is:

1. A method of providing centrifugal casting molds with high density refractory linings of predetermined thickness whereby castings of different outside diameters may be formed in the same imold by varying the thickness of the lining therein, comprising the steps of introducing into the mold a measured quantity of a flowable slurry of refractory material suspended in a liquid, said slurry containing a major proportion of relatively coarse refractory particles, a minor proportion of refractory fines and sufficient liquid to form a flowable suspension, the total amount of refractory material in said quantity being such as to form a lining having a predetermined thickness of at least 0.25 inch, rotating said mold at a speed and for a period of time sufficient to distribute the refractory constituents of said slurry by centrifugal force uniformly over the inner surface of the mold and to form a high density lining wherein the refractory fines are concentrated in a relatively thin layer at the inner surface of the lining and the coarser particles constitute a substantially thicker outer layer adjacent the mold wall, the speed and duration of rotation also being such as to cause the major portion of the liquid constituent of said slurry to collect as free liquid at the inner surface of the lining, maintaining the mold at a temperature less than 212 F. during the introduction of the slurry and the formation of the lining to avoid evaporation of the slurry liquid, and removing from the mold in liquid form the free liquid collected at the inner surface of the lining during rotation of the mold.

2. A method of providing centrifugal casting molds with high density refractory linings of predetermined thickness whereby castings of different outside diameters may be formed in the same mold by varying the thickness of the lining therein, comprising the steps of introducing into the mold a measured quantity of a flowable slurry of sand and silica fines suspended in Water, the total amount of sand and silica fines in said quantity being such as to form a lining having a predetermined thickness of at least 0.25 inch, rotating said mold at a speed and for a period of time suflicient to compact the solid particles of said slurry against the mold wall and to form a high density lining wherein the silica fines are concentrated in a smooth-surfaced layer at the inner portion of the lining and the sand constitutes the outer portion of the lining, the speed and duration of rotation also being such as to cause the major portion of the slurry water to collect as free liquid at the inner surface of the lining, maintaining the mold at a temperature less than 212 F. during the introduction of the slurry and the formation of the lining to avoid evaporation of the slurry water, removing from the mold in liquid form the free water of the slurry collected at the inner surface of the lining during rotation of the mold, and thereafter drying the mold lining to a substantially bonedry condition.

3. A method as defined in claim 2 wherein the drying step comprises passing heated air through the interior of the lined mold for a period of fro-m about four hours to about eight hours, the temperature of said heated air being not more than about 300 F. during the first hour 14 of said period and increasing thereafter to from about 400 to about 600 F.

4. A method of providing centrifugal casting molds with high density refractory linings of predetermined thickness whereby castings of diderent outside diameters may be formed in the same mold by varying the thickness of the lining therein, comprising the steps of forming an aqueous suspension of sand, silica fines and a suspending agent, introducing into the mold a measured quantity of said suspension in fiowable form, the total amount of sand and silica fines in said quantity being such as to form a lining having a predetermined thickness of at least 0.25 inch, rotating said mold at a speed and for a period of time sufiicient to distribute the solid particles of said suspension by centrifugal force uniformly over the inner surface of the mold and to form a high density lining wherein the silica fines and suspending agent are concentrated in a relatively thin smooth-surfaced layer at the inner surface of the lining and the sand constitutes a substantially thicker outer layer adjacent the mold wall, the speed and duration of rotation also being such as to cause the major portion of the Water of said suspension to collect freely at the inner surface of the lining, maintaining the mold at a temperature less than 212 F. during the introduction of the suspension and the formation of the lining to avoid evaporation of the water, removing from the mold in liquid form the free water collected at the inner surface of the lining during rotation of the mold, and thereafter drying the mold lining to evaporate substantially all of the chemically uncombined water remaining therein after removal of said free water.

5. A method of providing centrifugal casting molds with high density refractory linings of predetermined thickness whereby castings of different outside diameters may be formed in the same mold by varying the thickness of the lining therein, comprising the steps of forming an aqueous slurry of sand, silica flour and a suspending agent, introducing into the mold a measured quantity of said slurry in flowable form, the total amount of sand and silica flour in said quantity being such as to form a lining having a predetermined thickness of at least 0.25 inch, rotating said mold with its axis in a substantially horizontal position at a speed sufficient to distribute said slurry by centrifugal force uniformly over the inner surface of the mold to form a lining therein, continuing the rotation of said mold at such high speeds and for such a period of time that the major portions of the silica flour, suspending agent and water move to the inner portion of the lining and the sand becomes compacted against the inner surfaces of the mold wall, maintaining the mold wall at a temperature less than 212 F. during the introduction of the slurry and the rotation of the mold to avoid evaporation of the water, and removing from the mold in liquid form the water which collects at the inner surface of said lining during the high speed rotation of the mold.

6. A method as defined in claim 5 including the additional steps of Washing the inner surface of said lining after removal from the mold of the collected free water so as to remove from said surface any free residual suspending agent, and thereafter drying the mold lining to remove substantially all of the chemically uncombined water remaining therein.

7. A method of providing centrifugal casting molds with high density refractory linings of predetermined thickness whereby castings of different outside diameters may be formed in the same mold by varying the thickness of the lining therein, comprising the steps of forming an aqueous slurry consisting essentially of a major proportion of silica sand, a minor proportion of silica fines, a small amount of a suspending agent and sufficient water to form a relatively stable flo-wable suspension, introducing into the mold a measured quantity of said suspension, the total amount of sand and silica fines in said quantity being such as to form a lining having a predetermined thickness of at least 0.25 inch, rotating said mold with its axis in a substantially horizontal position at a speed and for a period of time sufficient to distribute the solid particles of said slurry by centrifugal force uniformly over the inner surface of the mold and to form a high density lining wherein the silica fines and suspending agent are concentrated in a relatively thin layer at the inner surface of the lining and the sand constitutes a substantially thicker outer layer adjacent the mold wall, the speed and duration of rotation also being such as to cause the major portion of the slurry water to collect as free liquid at the inner surface of the lining, maintaining the mold wall at a temperature less than 212 F. during the introduction of the slurry and the rotation of the mold to avoid evaporation of the water, and removing from the mold in liquid form the free water collected at the inner surface of the lining during rotation of the mold.

8. A method as defined in claim 7 wherein said slurry consists of at least 50% by weight of silica sand, from about 8% to about 30% by weight of silica fines, from about 0.5% to about 3.0% by Weight of a suspending agent, and from about 12% to about 30% by weight of water.

9. A method as defined in claim 7 wherein the speed of rotation of said mold during formation of the lining is such that the centrifugal force created at the inner surface of the lining is not less than about 20 times the force of gravity, and wherein the total time of rotation of the mold after introduction of the slurry i from about 0.5 to about 15.0 minutes.

10. A method of providing centrifugal casting molds having vent holes in the walls thereof with high density refractory linings of predetermined thickness whereby castings of different outside diameters may be formed in the same mold by varying the thickness of the lining therein, comprising the steps of introducing into the mold a measured quantity of a flowable slurry comprising a major proportion of sand and a minor proportion of silica fines suspended in Water, the total amount of sand and silica fines in said quantity being such as to form a lining having a predetermined thickness of at least 0.25 inch, rotating said mold with its axis in a substantially horizontal position at a relatively low speed until the vent holes in the wall of the mold become filled with sand from the slurry, then increasing the speed of rotation of said mold to a relatively high speed sufficient to distribute the remaining solid particles of said slurry by centrifugal force uniformly over the inner surface of the mold and to form a high density lining wherein the silica fines are concentrated in a relatively thin layer at the inner surface of the lining and the sand constitutes a substantially thicker outer layer adjacent the mold wall, said relatively high speed of rotation also being such as to cause the major portion of the slurry water to collect as free liquid at the inner surface of the lining, maintaining the inner surface of the mold wall at a temperature less than 212 F. during the introduction of the slurry and the rotation of the mold to avoid evaporation of the water, removing from the mold in liquid form the free water collected at the inner surface of the lining during said high speed rotation of the mold, and thereafter drying the mold lining to evaporate substantially all of the chemically uncombined water remaining therein after removal of said free water.

11. A method as defined in claim 10, wherein the relatively low speed of rotation is such that the centrifugal force exerted on the solid particles of the slurry is about 1.5 times the force of gravity, and the maximum speed of rotation is such that the centrifugal force created at the inner surface of the lining is not less than about 20 times the force of gravity.

References Cited in the file of this patent UNITED STATES PATENTS 2,128,404 Dunbeck Aug. 30, 1938 2,160,438 McCauley May 30, 1939 2,255,896 Projahn Sept. 16, 1941 2,399,606 Schuh et al. Apr. 30, 1946 2,623,809 Myers Dec. 30, 1952 2,731,690 Coupland Jan. 24, 1956 2,752,257 Bradley et al. June 26, 1956 2,852,399 Bellezanne Sept. 16, 1958 2,857,285 Stoddard Oct. 2], 1958 2,946,104 Martin July 26, 1960 

1. A METHOD OF PROVIDING CENTRIFUGAL CASTING MOLDS WITH HIGH DENSITY REFRACTORY LININGS OF PREDETERMINED THICKNESS WHEREBY CASTINGS OF DIFFERENT OUTSIDE DIAMETERS MAY BE FORMED IN THE SAME MOLD BY VARYING THE THICKNESS OF THE LINING THEREIN, COMPRISING THE STEPS OF INTRODUCING INTO THE MOLD A MEASURED QUNTITY OF A FLOWABLE SLURRY OF REFRACTORY MATERIAL SUSPENDED IN A LIQUID, SAID SLURRY CONTAINING A MAJOR PROPORTION OF RELATIVELY COARSE REFRACTORY PARTICLES, A MINOR PROPORTION OF REFRACTROY FINES AND SUFFICIENT TO FORM A FLOWABLE SUSPENSION, THE TOTAL AMOUNT OF REFRACTORY MATERIAL IN SAID QUANTITY BEING SUCH AS TO FORM A LINING HAVING A PREDERERMINED THICKNESS OF AT LEAST 0.25 INCH, ROTATING 